Reverse osmosis is a well-known process for purification of saline water. In this process, a pressure in excess of the osmotic pressure of the saline water feed solution is applied to the feed solution to separate purified water by use of a semipermeable permselective membrane. Purified water is thereby caused to diffuse through the membrane while salt and other impurities are retained by the membrane.
Permselective membranes include composite membranes that include a separating layer on a supporting microporous substrate. The substrate is typically supported on a support fabric to impart mechanical strength to the membrane. Permselective membranes suitable for use in reverse osmosis are available in various forms and configurations. Flat sheet, tubular and hollow fiber membranes are well-known in the art. These membranes can also vary in morphology. Homogenous and asymmetric membranes are operable, as well as thin film composites.
Permselective membranes are available in the form of multilayer structures that include a separating layer superimposed on a microporous polysulfone substrate layer. Membrane separating layers which may be employed include polyamides, polyphenylene esters, and polysulfonamides.
Polyamide discriminating layers are well-known in the art. The polyamide can be aliphatic or aromatic and may be crosslinked. Polyamide membranes may be made by the interfacial reaction of a cycloaliphatic diamine with isophthaloyl chloride, trimesoyl chloride or mixtures of these acid chlorides. Polyamide membranes also may be made by reaction of m-phenylenediamine and cyclohexane-1,3,5-tricarbonyl chloride. The polyamide discriminating layer also may be made by reaction of aromatic polyamines having at least two primary amines on an aromatic nucleus and aromatic polyfunctional acyl halides having an average of more than two acyl halide groups on an aromatic nucleus.
These prior art membranes have generally been useful as reverse osmosis membranes. These membranes, however, have been prone to deficiencies such as short useful life, low flux, and low salt rejection. A need therefore exists for improved reverse osmosis membranes which show both high rates of salt rejection while providing improved rates of flux.